1. Field of the Invention
The present invention relates to optical apparatus employed to project color transparencies, and more particularly, to such optical apparatus having means for suppressing the appearance of scratches in the transparency.
2. Description of the Problem
Equipment in which color transparencies such as color slides, color negatives, or color movie film are scanned or projected, are well known. Such equipment includes graphic arts scanners, telecine and slide scanning apparatus, and photographic printers. FIG. 1 shows one example of a prior art color film scanner. In the film scanner shown in FIG. 1, a scanning raster pattern generated by a CRT 10 is imaged onto a color transparency 12 by an objective lens 14. The light emerging from the color transparency is relayed by a condenser lens 16 to a pair of dichroic mirrors 18 and 20 employed to separate the light into three primary colored beams. The respective colored beams are intercepted by three photomultiplier tubes 22, 24 and 26 to produce three additive primary color signals R, G, and B respectively.
The reflection and transmission characteristics of dichroic mirrors are a strong function of the angle of incidence of the light striking the mirrors. As a result, one of the problems with prior art scanning arrangements like that shown in FIG. 1, is that the peripheral portions of the raster exhibit a color shaft with respect to the central portion. Various approaches have been employed to correct this problem, including the use of subtractive trimming filters in front of the photomultiplier tubes, and the use of extra condenser lenses to bend the off axis beams closer to the optical axis.
Another problem encountered in prior art film scanners, as well as in projection printers, is caused by scratches in the transparency to be projected. The appearance of a scratch is amplified in the scanning or projecting process and results in a white or dark line in the resulting picture, depending upon whether the transparency is a negative or a positive. The reason for this can be appreciated from FIG. 2, which shows a partial cross section of a piece of transparent film 28, bearing a scratch with facets 30 and 32. Each facet of the scratch acts like a prism that refracts light passing through the film. For example, a ray of light 34 which would normally pass through the film, exiting along the dotted line 36, is refracted by scratch facet 30 into the direction 36'. Similarly, a ray of light 38 which would normally pass through the film and exit along dotted line 40, is refracted by scratch facet 32 into the direction 40'. Much of the light refracted in this manner by scratches escapes from the optical path of the scanning or projection printing apparatus and is never detected by the photosensors in the case of film scanners or by the photosensitive medium in the case of projection printers.
One prior art solution to this problem, in the case of projection printers, was to employ a diffuse source of illumination so that for each ray that is refracted by a scratch and escapes the optical path of the apparatus, another ray that normally would have escaped the optical path is refracted into the optical path, thereby "filling-in" for the light normally lost by the scratch. For projection printers, this solution is inefficient since much light is wasted. The method cannot be used at all in film scanners, which by their very nature require a specular rather than a diffuse source of illumination.
One prior art approach to scratch suppression that works in both projection printers and scanners is to cover the scratch with an index matching liquid in a so-called liquid gate. This method is effective but it is both messy and costly.
It has been suggested in the prior art that the appearance of scratches may be suppressed in a film scanner by photoelectronically detecting the light refracted away from the optical path by a scratch, and substituting the scanned information from the area of the scratch with the information from adjacent regions. This approach involves the use of complicated electronics and sophisticated sensing and signal processing hardware, and is not suitable for use with projection printers.
A further approach employed in both film scanners and projection printers has been to surround the optical system with reflecting surfaces, to either (1) redirect light scattered by scratches into the photosensors in the case of film scanners, or (2) to provide off-axis "fill-in" light in the case of projection printers. This approach, when used with film scanners, can result in the appearance of a properly exposed but color-shifted scratch, because the light refracted by the facets of the scratch encounters the dichroic filter in the scanner at angles different from that of the unrefracted light.
In light of the foregoing, the challenge faced by the present inventor was to provide a scratch suppressing optical projection arrangement that would avoid the problems with the prior art scratch suppressing schemes especially the problem of introducing color shifts into the suppressed scratches.